CN102396132B - Adaptive impedance tuning in wireless power transmission - Google Patents

Abstract

Exemplary embodiments are directed to wireless power. A wireless power receiver includes a receive antenna for coupling with near-field radiation in a coupling-mode region generated by a transmit antenna operating at a resonant frequency. The receive antenna generates an RF signal when coupled to the near-field radiation, and a rectifier converts the RF signal to a DC input signal. A DC-DC converter coupled to the DC input signal generates a DC output signal. A pulse modulator generates a pulse width modulated signal to the DC-DC converter to adjust a DC impedance of the wireless power receiver by modifying a duty cycle of the pulse width modulated signal in response to at least one of a voltage of the DC input signal, a current of the DC input signal, a voltage of the DC output signal, and a current of the DC output signal.

Description

Adaptive Impedance Tuning in Wireless Power Transmission

Claim of priority under 35 U.S.C. §119

This application claims priority under 35 U.S.C. § 119(e) to the following applications:

U.S. Provisional Patent Application 61/162,157, entitled "WIRELESS POWER IMPEDANCE CONTROL," filed March 20, 2009, and

US Provisional Patent Application 61/176,468, entitled "DC-BASED ADAPTIVE TUNING," filed May 7, 2009.

technical field

The present invention relates generally to wireless power transfer, and more particularly to devices, systems and methods related to adaptively tuning impedance in a receiver device to improve wireless power transfer.

Background technique

Typically, each battery powered device (eg, wireless electronic device) requires its own charger and power source, typically an alternating current (AC) power outlet. Such wired configurations become inconvenient to use when many devices need to be charged.

Methods are being developed that use over-the-air or wireless power transmission between a transmitter and a receiver coupled to an electronic device to be charged. Such methods are generally divided into two categories. One is based on the coupling of plane wave radiation (also called far-field radiation) between the transmit antenna and the receive antenna on the device to be charged, the receive antenna collects the radiated power and rectifies it for charging the battery . Antennas generally have a resonant length in order to improve coupling efficiency. This approach suffers from the fact that the electrical coupling decays rapidly with the distance between the antennas. Thus, charging over reasonable distances (eg, less than 1 to 2 meters) becomes difficult. Additionally, since the transmitting system radiates plane waves, unintentional radiation can interfere with other systems if not properly controlled through filtering.

Other methods for wireless energy transmission technology are based on inductive coupling between a transmitting antenna embedded in, for example, a "charging" pad or surface, and a receiving antenna (plus a rectification circuit) embedded in the electronic device to be charged. This approach has the disadvantage that the spacing between the transmit and receive antennas must be very close (eg, within a few thousandths of a meter). While this method does have the ability to charge multiple devices in the same area at the same time, this area is typically very small and requires the user to precisely position the devices into a specific area.

In a wireless power transfer system, efficiency is important due to losses that occur during wireless power transmission. Since wireless power transmission is generally less efficient than wired transfer, efficiency has received more attention in the wireless power transfer environment.

As a result, when attempting to provide power to one or more wireless charging devices, changes in the coupling between the transmit antenna and the receive antenna need to be accommodated to optimize or otherwise adjust the power delivered to the receiver device coupled to the receive antenna methods and equipment.

Contents of the invention

In one novel aspect, a wireless power receiver is provided. The receiver includes a rectifier coupled to an antenna. The antenna is configured to receive wireless power signals. The rectifier is configured to convert the wireless power signal to a DC input signal. The receiver also includes a DC-DC converter configured to generate a DC output signal based in part on the DC input signal and the pulse width modulated signal. The receiver further includes a pulse modulator configured to adjust the wireless power by modifying a duty cycle of the pulse width modulated signal to the DC-DC converter based in part on the DC input signal AC impedance of the receiver.

In another novel aspect, a method is provided. The method includes receiving a wireless power signal. The method includes rectifying the wireless power signal into a DC input signal. The method further includes converting the DC input signal to a DC output signal. The method also includes modifying an AC impedance of the wireless power receiver by adjusting a power output of the DC output signal based in part on the DC input signal.

In yet another novel aspect, a wireless power receiver is provided. The receiver includes means for receiving wireless power signals. The receiver also includes means for rectifying the wireless power signal into a DC input signal. The receiver further comprises means for converting the DC input signal into a DC output signal. The receiver also includes means for modifying the AC impedance of the wireless power receiver, including means for adjusting the input signal from the DC input signal for converting the DC input signal to a DC output signal based in part on the DC input signal. The DC output signal of the device is the power output of the device.

In other novel aspects, a computer-readable storage medium embodying instructions executable by a processor of a device is provided. The instructions cause the device to couple near-field radiation in a coupling mode region to generate a wireless power signal for a wireless power receiver. The instructions further cause the device to rectify the wireless power signal into a DC input signal; convert the DC input signal into a DC output signal. The instructions also cause the apparatus to modify an AC impedance of the wireless power receiver by adjusting a power output of the DC output signal from DC-DC based in part on the DC input signal.

Description of drawings

Figure 1 shows a simplified block diagram of a wireless power transfer system.

2 shows a simplified schematic diagram of a wireless power transfer system.

Figure 3 shows a schematic diagram of a loop antenna for use in an exemplary embodiment of the invention.

4 is a simplified block diagram of a transmitter according to an exemplary embodiment of the invention. the

5 is a simplified block diagram of a receiver according to an exemplary embodiment of the invention. the

6 shows a schematic diagram of a transmit circuit and a receive circuit showing coupling and adjustable DC load therebetween.

7A-7B show Smith charts illustrating changes in the input impedance of a coupled coil pair in response to changes in DC impedance at the receiver device.

8A-8B show amplitude curves showing improved coupling between coupling coil pairs in response to changes in DC impedance at the receiver device.

9A-9B show simplified schematic diagrams of receiver devices illustrating exemplary embodiments for adjusting DC impedance at the receiver device.

10A-10D show simplified schematic diagrams of receiver devices illustrating exemplary embodiments for adjusting DC impedance at the receiver device using a pulse width modulation converter.

11 illustrates various input and output parameters that may be used in adjusting the DC impedance at the receiver device.

Detailed ways

The word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. the

The detailed description set forth below in conjunction with the accompanying drawings is intended as a description of exemplary embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. The term "exemplary" is used throughout this description to mean "serving as an example, instance or illustration" and should not necessarily be construed as preferred or advantageous over other exemplary embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the exemplary embodiments of the invention. It will be readily apparent to those skilled in the art that the exemplary embodiments of the invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the novelty of the exemplary embodiments presented herein.

The term "wireless power" is used herein to refer to any form of energy associated with electric, magnetic, electromagnetic, or otherwise transmitted between a transmitter and a receiver without the use of physical electromagnetic conductors.

FIG. 1 illustrates a wireless transmission or charging system 100 according to various exemplary embodiments of the invention. Input power 102 is provided to a transmitter 104 for generating a radiated field 106 for providing energy transfer. Receiver 108 is coupled to radiated field 106 and generates output power 110 for storage or consumption by a device (not shown) coupled to output power 110 . The transmitter 104 and the receiver 108 are separated by a distance 112 . In an exemplary embodiment, the transmitter 104 and receiver 108 are configured according to a mutual resonance relationship, and when the receiver 108 is located in the "near field" of the radiated field 106, when the resonant frequency of the receiver 108 and the transmitter 104 The transmission loss between the transmitter 104 and the receiver 108 is minimal when the resonant frequencies of are very close.

The transmitter 104 further includes a transmit antenna 114 for providing means for energy transmission, and the receiver 108 further includes a receive antenna 118 for providing means for energy reception. The transmit and receive antennas are sized according to the application and the device with which it will be associated. As stated, efficient energy transfer occurs by coupling most of the energy in the near field of the transmit antenna to the receive antenna rather than propagating most of the energy in the form of electromagnetic waves to the far field. While in this near field, a coupling mode may form between the transmit antenna 114 and the receive antenna 118 . The region around antennas 114 and 118 where this near-field coupling can occur is referred to herein as the coupling-mode region.

2 shows a simplified schematic diagram of a wireless power transfer system. The transmitter 104 includes an oscillator 122 , a power amplifier 124 and a filter and matching circuit 126 . The oscillator is configured to generate a desired frequency, which is adjustable in response to an adjustment signal 123 . The oscillator signal may be amplified by the power amplifier 124 with an amplification amount responsive to the control signal 125 . A filter and matching circuit 126 may be included to filter out harmonics or other unwanted frequencies and to match the impedance of the transmitter 104 to the transmit antenna 114 .

The receiver 108 may include a matching circuit 132 and a rectifier and switching circuit 134 to generate a DC power output to charge a battery 136 (as shown in FIG. 2 ) or to power a device (not shown) coupled to the receiver. A matching circuit 132 may be included to match the impedance of the receiver 108 to the receive antenna 118 . Receiver 108 and transmitter 104 may communicate over a separate communication channel 119 (eg, Bluetooth, zigbee, cellular, etc.).

As illustrated in FIG. 3, the antenna used in the exemplary embodiment may be configured as a "loop" antenna 150, which may also be referred to herein as a "magnetic" antenna. Loop antennas may be configured to include an air core or a physical core (eg, a ferrite core). Air core loop antennas may be more tolerant to extraneous physical devices placed near the core. Furthermore, air core loop antennas allow other components to be placed within the core area. Additionally, the air core ring may more easily enable placement of the receive antenna 118 (FIG. 2) within the plane of the transmit antenna 114 (FIG. 2), where the coupled-mode region of the transmit antenna 114 (FIG. 2) may be more robust .

As stated, efficient energy transfer between the transmitter 104 and the receiver 108 occurs during a matched or nearly matched resonance between the transmitter 104 and the receiver 108 . However, even when the resonance between the transmitter 104 and the receiver 108 is mismatched, energy may be transferred less efficiently. The transfer of energy occurs by coupling energy from the near field of the transmitting antenna to the receiving antenna residing in the neighborhood where this near field is established, rather than propagating the energy from the transmitting antenna into free space.

The resonant frequency of a loop or magnetic antenna is based on inductance and capacitance. Inductance in a loop antenna is generally simply the inductance created by the loop, whereas capacitance is generally added to the loop antenna's inductance to create a resonant structure at the desired resonant frequency. As a non-limiting example, capacitor 152 and capacitor 154 may be added to the antenna to create a resonant circuit that produces resonant signal 156 . Thus, for larger diameter loop antennas, the amount of capacitance required to induce resonance decreases as the diameter or inductance of the loop antenna increases. Furthermore, as the diameter of the loop antenna or magnetic antenna increases, the effective energy transfer area in the near field increases. Of course, other resonant circuits are possible. As another non-limiting example, a capacitor may be placed in parallel between the two terminals of the loop antenna. Additionally, those skilled in the art will recognize that for a transmit antenna, the resonant signal 156 may be an input to the loop antenna 150 .

An exemplary embodiment of the invention includes coupling power between two antennas that are in the near field of each other. As stated, the near field is the area around an antenna where an electromagnetic field exists but may not propagate or radiate away from the antenna. The electromagnetic field is generally confined to a volume in the vicinity of the physical volume of the antenna. In an exemplary embodiment of the present invention, magnetic type antennas (e.g., single-turn loop antennas and multi-turn loop antennas) are used for both the transmit (Tx) and receive (Rx) antenna systems because the magnetic type Magnetic antennas tend to have a higher magnetic near-field amplitude than the electric near-field of an antenna such as a small dipole antenna. This allows for potentially higher coupling between the pair of antennas. Additionally, "electric" antennas (eg, dipoles and monopoles) or a combination of magnetic and electric antennas are also contemplated.

Tx antennas can be operated at sufficiently low frequencies and with sufficiently large antenna sizes to achieve good coupling to small receive antennas at distances significantly greater than allowed by the earlier mentioned far-field and inductive approaches (eg, >-4dB). If the transmit antenna is sized correctly, high coupling levels (e.g., -1 to -4dB).

FIG. 4 is a simplified block diagram of a transmitter 200 according to an exemplary embodiment of the present invention. The transmitter 200 includes a transmit circuit 202 and a transmit antenna 204. In general, transmit circuitry 202 provides RF power to transmit antenna 204 by providing an oscillating signal that causes near-field energy to be generated around transmit antenna 204 . For example, transmitter 200 may operate in the 13.56 MHz ISM band.

Exemplary transmit circuitry 202 includes a fixed impedance matching circuit 206 for matching the impedance (e.g., 50 ohms) of transmit circuitry 202 to transmit antenna 204; and a low-pass filter (LPF) 208 configured to Wave emission is reduced to a level that prevents self-interference of devices coupled to receiver 108 (FIG. 1). Other exemplary embodiments of matching circuits may include inductors and transformers. Other exemplary embodiments of low-pass filters may include different filter topologies including, but not limited to, notch filters that attenuate certain frequencies while passing other frequencies, and may include adaptive impedance matching, which may be based on Measurable emission metrics such as output power to an antenna or DC current drawn by a power amplifier vary. Transmit circuitry 202 further includes a power amplifier 210 configured to drive an RF signal as determined by an oscillator 212 (also referred to herein as a signal generator). The transmit circuitry may comprise discrete devices or circuits, or may comprise an integrated assembly. An exemplary RF power output from transmit antenna 204 may be approximately 2.5 to 8.0 watts.

The transmit circuit 202 further includes a controller 214 for enabling the oscillator 212 during a transmit phase (or duty cycle) for a particular receiver, for adjusting the frequency of the oscillator, and for adjusting the output power A communication protocol for interacting with a neighboring device via a receiver attached to the neighboring device is implemented at the level. The controller 214 is also used to determine the change in impedance at the transmit antenna 204 due to the change in the coupling mode region due to the receiver placed therein.

Transmit circuitry 202 may further include load sensing circuitry 216 for detecting the presence or absence of active receivers in the vicinity of the near field generated by transmit antenna 204 . For example, load sensing circuit 216 monitors the current flowing to power amplifier 210 , which is affected by the presence or absence of an active receiver near the near field generated by transmit antenna 204 . Detection of a change in loading on power amplifier 210 is monitored by controller 214 for use in determining whether to enable oscillator 212 for transmitting energy to communicate with an active receiver.

Transmit antenna 204 may be implemented as an antenna strip with a thickness, width, and metal type selected to keep resistive losses low. In conventional implementations, the transmit antenna 204 may generally be configured for association with a larger structure such as a table, cushion, lamp, or other less portable configuration. Thus, transmit antenna 204 generally will not require "several turns" in order to be of a practical size. An exemplary implementation of transmit antenna 204 may be "electrically small" (ie, a fraction of a wavelength) and tuned to resonate at lower usable frequencies by using capacitors to define the resonant frequency. In an exemplary application where the transmit antenna 204 may be relatively large in diameter or side length (if a square loop) (eg, 0.50 meters) relative to the receive antenna, the transmit antenna 204 will not necessarily require a large number of turns to obtain reasonable capacitor in order to resonate the transmit antenna at the desired frequency. .

Transmitter 200 may gather and track information regarding the whereabouts and status of receiver devices that may be associated with transmitter 200 . Accordingly, the transmitter circuit 202 may include a presence detector 280, an enclosure detector 290, or a combination thereof connected to the controller 214 (also referred to herein as a processor). Controller 214 may adjust the amount of power delivered by amplifier 210 in response to the presence signals from presence detector 280 and enclosure detector 290 . The transmitter can receive power via a number of power sources, such as an AC-DC converter (not shown) to convert conventional AC power present in the building, to convert conventional DC power to a power source suitable for the transmitter. 200 voltage DC-DC converter (not shown), or the transmitter may receive power directly from a conventional DC power source (not shown).

As a non-limiting example, presence detector 280 may be a motion detector to sense the initial presence of a device to be charged inserted into the transmitter's coverage area. After detection, the transmitter can be turned on and RF power received by the device can be used to toggle a switch on the receiver device in a predetermined manner, which in turn causes a change in the transmitter's driving point impedance.

As another non-limiting example, presence detector 280 may be a detector capable of detecting a human being, such as through infrared detection, motion detection, or other suitable means. In some demonstrative embodiments, there may be regulations that limit the amount of power that a transmit antenna can transmit at a particular frequency. In some cases, these regulations are intended to protect humans from electromagnetic radiation. However, there may be environments where transmit antennas are placed in areas that are not occupied by humans or that are not frequently occupied by humans (eg, garages, factory areas, workshops, etc.). If these environments are free of humans, it may be permissible to increase the power output of the transmit antenna above normal power constraints regulations. In other words, the controller 214 may adjust the power output of the transmit antenna 204 to a regulatory level or lower in response to the presence of a human, and when the human is outside the electromagnetic field regulatory distance from the transmit antenna 204, turn the transmit antenna 204 The power output is adjusted to go to a level above the regulatory level. Also, the presence detector 280 may be a detector capable of detecting objects placed in the area of the transmitting antenna. This detection can be used to reduce or stop power output when an object that is not intended to receive wireless power and that could be damaged by a magnetic field is placed near the transmitting antenna.

As a non-limiting example, the enclosure detector 290 (also referred to herein as an enclosed compartment detector or an enclosed space detector) may be a device such as a sensing switch for use in determining when the enclosure is in closed or open state. Increases the power level of the transmitter when the transmitter is in a closed enclosure.

In an exemplary embodiment, a method by which the transmitter 200 does not remain on indefinitely may be used. In this case, the transmitter 200 may be programmed to turn off after a user-determined amount of time. This feature prevents the transmitter 200, especially the power amplifier 210, from running for long periods of time after the wireless devices in its vicinity are fully charged. This event may be due to a failure of the circuitry used to detect the signal sent from the repeater or receiving coil indicating that the device is full. To prevent the transmitter 200 from automatically shutting down when another device is placed in its vicinity, the transmitter 200 auto-shutdown feature may only be activated after a set period of lack of motion in its perimeter is detected. The user may be able to determine the inactivity time interval, and change the inactivity time interval if desired. As a non-limiting example, the time interval may be longer than that required to fully charge a particular type of wireless device assuming the device is initially fully discharged.

FIG. 5 is a simplified block diagram of a receiver 300 according to an exemplary embodiment of the present invention. The receiver 300 includes a receiving circuit 302 and a receiving antenna 304 . Receiver 300 is further coupled to device 350 for providing received power to device 350 . It should be noted that receiver 300 is illustrated as being external to device 350 , but it could be integrated into device 350 . Typically, the energy propagates wirelessly to receive antenna 304 and is then coupled to device 350 via receive circuitry 302 .

Receive antenna 304 is tuned to resonate at or near the same frequency as transmit antenna 204 (FIG. 4). Receive antenna 304 may be similarly sized as transmit antenna 204 , or may be sized differently based on the size of associated device 350 . For example, device 350 may be a portable electronic device having a diameter or length dimension that is smaller than the diameter or length of the transmit antenna 204 . In such an example, receive antenna 304 may be implemented as a multi-turn antenna in order to reduce the capacitance value of a tuning capacitor (not shown) and increase the impedance of the receive antenna. For example, receive antenna 304 may be placed around the substantial circumference of device 350 in order to maximize the antenna diameter and reduce the number of loop turns (ie, coils) and inter-coil capacitance of the receive antenna.

Receive circuit 302 provides impedance matching with receive antenna 304 . Receive circuitry 302 includes power conversion circuitry 306 for converting received RF energy into charging power for use by device 350 . Power conversion circuitry 306 includes an RF-DC converter 308 and may also include a DC-DC converter 310 . RF-DC converter 308 rectifies the RF energy signal received at receive antenna 304 into non-alternating electrical power, and DC-DC converter 310 converts the rectified RF energy signal into an energy potential compatible with device 350 (e.g., Voltage). A variety of RF-DC converters are contemplated, including partial and full rectifiers, regulators, bridges, multipliers, and linear and switching converters.

The receiving circuit 302 may further include a switching circuit 312 for connecting the receiving antenna 304 to the power conversion circuit 306 or for disconnecting the power conversion circuit 306 . Disconnecting the receive antenna 304 from the power conversion circuit 306 not only discontinues charging the device 350, but also changes the "load" seen by the transmitter 200 (FIG. 2), which can be used to "shade" the receiver from being detected. Transmitter sees.

As disclosed above, the transmitter 200 includes a load sensing circuit 216 that detects fluctuations in the bias current provided to the transmitter power amplifier 210 . Accordingly, the transmitter 200 has a mechanism for determining when a receiver is present in the near field of the transmitter.

When multiple receivers 300 are present in the near field of a transmitter, it may be desirable to time multiplex the loading and unloading of one or more receivers to enable other receivers to couple more efficiently to the transmitter. A receiver may also be masked to eliminate coupling to other nearby receivers or to reduce loading on nearby transmitters. This "offloading" of the receiver is also referred to herein as "cloaking." Furthermore, this switching between unloading and loading, controlled by the receiver 300 and detected by the transmitter 200, provides a communication mechanism from the receiver 300 to the transmitter 200, as explained more fully below. Additionally, a protocol enabling the sending of messages from the receiver 300 to the transmitter 200 may be associated with the handover. For example, the switching speed may be about 100 microseconds.

In an exemplary embodiment, communication between the transmitter and receiver involves device sensing and charging control mechanisms rather than conventional two-way communication. In other words, the transmitter uses on/off keying of the transmitted signal to adjust whether energy is available in the near field. The receiver interprets these energy changes as messages from the transmitter. From the receiver side, the receiver uses tuning and detuning of the receive antenna to adjust how much power is being received from the near field. The transmitter can detect this difference in power used from the near field, and interpret these changes as messages from the receiver.

Receive circuitry 302 may further include signaling detector and beacon circuitry 314 to identify received energy fluctuations, which may correspond to informative signaling from the transmitter to the receiver. In addition, the signaling and beaconing circuit 314 can also be used to detect the transmission of reduced RF signal energy (i.e., a beacon signal) and rectify the reduced RF signal energy to nominal power for waking up the RF signal within the receiving circuit 302. Unpowered or depleted circuitry in order to configure receiving circuitry 302 for wireless charging.

Receive circuitry 302 further includes a processor 316 for coordinating the processing of receiver 300 described herein (including control of switching circuitry 312 described herein). Covering of receiver 300 may also occur upon occurrence of other events, including detection of an external wired charging source (eg, wall/USB power) providing charging power to device 350 . In addition to controlling masking of the receiver, the processor 316 may also monitor the beacon circuit 314 to determine beacon status and extract messages sent from the transmitter. Processor 316 may also tune DC-DC converter 310 for improved performance.

In some demonstrative embodiments, receive circuitry 320 may signal the power requirement in the form of, for example, a desired power level, a maximum power level, a desired current level, a maximum current level, a desired voltage level, and a maximum voltage level. sent to the transmitter. Based on these levels and the actual amount of power received from the transmitter, processor 316 may adjust the operation of DC-DC converter 310 to adjust its output in the form of adjusted current levels, adjusted voltage levels, or a combination thereof.

Exemplary embodiments of the present invention are directed to circuits and adjustment mechanisms that allow adjustment of the load impedance of a receive antenna terminating a receiver device in a manner that can compensate for changes in coupling effects between the transmit and receive antennas.

The current scheme for adjusting the load impedance is based on the use of RF components. These RF components include tuners based on switchable fixed capacitors and inductors, voltage variable capacitors (eg, ferroelectric, microelectromechanical systems (MEMS), and varactor diodes). A switchable fixed capacitor and inductor approach may have too many ohmic losses to be practical for a charging system. Variable tuners based on ferroelectric devices and MEMS voltage variable capacitors may not be commercially viable at this time. Varactor based tuners may not be able to handle the RF power expected in wireless power applications.

As described above, wireless charging systems typically include a transmit antenna (i.e., transmit coupling coil) that transmits RF energy to one or more receive antennas (i.e., receive coupling coils) that The antenna is embedded in the receiver device to be charged or otherwise supplied with power. The received energy is rectified, conditioned, and delivered to the device's battery or other operating circuitry. These antennas typically operate at low frequencies where the antennas are electrically compact so as to couple magnetically without radiating power.

These small antennas achieve better coupling efficiency when the two coils are resonant (ie, when both are tuned to the frequency used to transfer power from one antenna to the other). Unfortunately, while efficient power transfer is an important aspect of any wireless power transfer scheme, a side effect of using small, resonantly coupled antennas is that the resulting bandwidth is sometimes quite small, making the antenna susceptible to detuning and efficiency can be susceptible to large swings. loss. Another problem with using small loosely coupled resonant antennas is when the receive antenna is moved around relative to the transmit antenna (e.g. at different locations on the charging pad), or when multiple devices to be charged are placed close to each other on the charging pad , the mutual coupling between the two antennas will vary. These placement changes will change the coupling between the transmit and receive coils and cause changes in the impedance seen at the transmit antenna, resulting in inefficient power transfer between the transmit and receive antennas in the charging system. Many of these problems can be corrected, or at least largely reduced, by varying the RF load resistance presented to the receive antenna.

In varying the RF load resistance in order to affect the change in the impedance seen by the transmit amplifier, it is well known that this impedance seen by the power supply can be resistive, reactive way or a combination thereof. To maximize the efficiency of the system, it is best to vary only real values (ie, resistive values) and keep the reactive value of this input impedance as constant as possible. Although it is possible to compensate for reactive changes, this can greatly increase the complexity of the overall system. It can be shown that there is a matching circuit that meets the goal of maximum power transfer over any range of resistive load variation. The matching circuit may be a tuned (resonant) transformer, which is simply an extension of the resonant transmit and receive antennas used to transfer power. In the following discussion, it is assumed that this form of matching circuit is used.

FIG. 6 shows a schematic diagram of a transmit circuit and a receive circuit showing coupling and an adjustable DC load 450 therebetween. As shown in FIG. 6, the charging system 405 can be characterized by a coupled coil transformer model 430, where the transmitter electronics are connected to the primary coil 432 (i.e., the transmit antenna), and the rectifier/regulator electronics on the receiver side are connected to the secondary coil 432. Coil 434 (ie, receive antenna).

Driver 410 generates an oscillating signal at a desired resonant frequency (eg, about 13.56 MHz). As an example, this driver 41 may be configured as a class E driver as illustrated in FIG. 6 . Low pass matching circuit 420 filters and impedance matches the signal from driver 410 to transmit antenna 432 of coupled coil transformer model 430 .

Energy is transferred to receive antenna 434 of coupled-coil transformer model 430 via near-field radiation. The oscillating signal coupled to receive antenna 434 is coupled to impedance matching and rectifier circuit 440 to provide AC impedance matching for receive antenna 434 and to rectify the oscillating signal into a substantially DC signal. DC-DC converter 450 converts the DC signal from rectifier 440 to a DC output that can be used by circuitry on a receiver device (not shown). DC-DC converter 450 is also configured to adjust the DC impedance seen by rectifier 440 , which in turn adjusts the overall AC impedance to the input of rectifier 440 . As a result, a change in the DC impedance at the input of DC-DC converter 450 may result in a better match to the impedance of receive antenna 434 and better mutual coupling between receive antenna 434 and transmit antenna 432 .

The self-inductance (Ltx and Lrx), mutual inductance (m) and loss resistance of the transformer model 430 can be derived from the measured or simulated coupling characteristics of the antenna pair.

It can be shown that, given the mutual inductance (m) and resistive losses (R1 and R2 for the transmit and receive antennas, respectively), there is an optimal load for the receive antenna that will maximize power transfer efficiency. This optimal load can be defined as:

R _eff =R1*[1+(Ω*m) ² /(R1*R2)] ⁵ .

Typically, R _eff may be in the range of 1 ohm to 20 ohms. By using DC load control, the RF load seen by the receive coil 434 can be set to its most effective value since the mutual inductance (m) varies for the reasons described above.

Another use of controlling the RF load is that changes in the load can be used to control the power delivered to the receiver device. This may come at the expense of some efficiency, but enables maximum use of available power when servicing a mixture of wireless devices in various states of charge.

Yet another use for controlling RF loading is that variations in the load can be used to widen the bandwidth of the transfer function, the result of which depends on the relationship between a very low or reactive impedance transmit power amplifier 410 (typically a wireless variable amplifier) and transmit antenna 432. The matching network 420 between. If the input matching circuit includes a third tuned inductor (not shown) that is mutually coupled with the TX antenna 432, then this bandwidth adjustment works best over a range of large mutual inductance (m) and load variations. In this case, if the power amplifier has very low source impedance, the bandwidth will increase linearly with increasing RF load resistance.

Existing wireless charging systems seem bandwidth-independent because the signal bandwidth allowed by the FCC is quite small. As stated before, changing the load to widen the bandwidth can reduce the efficiency slightly from its maximum value, but this can be used to maintain a functional charging system when increased bandwidth may be required. While not a substantially desirable option in wireless charging where high efficiency is required, this bandwidth-extending effect can be applied in short-range communication systems where efficiency is not critical.

Using low inductance to inter-couple with transmit antenna 432 provides significant system advantages over more common passive matching. This input in series with the tuned DC-DC converter 450 causes a second impedance inversion, the first impedance inversion being between the transmit and receive antennas (432 and 434). As a result, when the load impedance increases, the input impedance increases. This simply allows the load to "cloak" the receiver from the transmitter's view by raising the receiver's load impedance. This effect can be restated so that the input conductance is a linear function of the load conductance.

Without this covering feature, the load from the receiver would be presented as a short circuit for covering, using a mechanism such as element 312 discussed above with reference to FIG. 5 . As a result, a charging pad in the absence of a receiver device will appear as a highly tuned short circuit rather than an open circuit. Furthermore, when there are multiple uncovered loads, the total input conductance of the transmit antenna 432 will be the sum of the individual conductances of the receive antenna 434, and power will be distributed according to their relative values.

A further advantage of tuned input transformer matching is that the resulting input/output admittance is real at the center (resonant) frequency and is "flat" with respect to frequency. Therefore, the first-order changes of circuit parameters have less influence on the power transfer process.

7A-7B show Smith charts illustrating changes in the input impedance of a coupled coil pair (without adding inductive matching) in response to changes in DC impedance at the receiver device. In FIGS. 7A and 7B , bold circles 510 and 520 indicate constant resistance circles, respectively.

7A and 6, a DC impedance _Rdc of about 10.2 ohms at the input of the DC-DC converter 450 produces a composite input impedance of about 50 ohms at the transmit antenna 432, with minimal reactance. 7B and 6, a DC impedance _Rdc of approximately 80 ohms at the input of the DC-DC converter 450 produces a composite input impedance at the transmit antenna 432 of much less than 50 ohms, and minimal reactance.

8A and 8B show amplitude curves (530 and 540, respectively) showing improved coupling between coupling coil pairs in response to changes in DC impedance at the receiver device. In FIG. 8A, the amplitude at the center frequency of 13.56 MHz is about -4.886 dB. After adjusting the input impedance of the DC-DC converter 450 (FIG. 6), the amplitude at the center frequency of 13.56 MHz improves to about -3.225 dB, resulting in better coupling between the receive and transmit antennas, which results in more power is passed to the receiving antenna.

9A-9B show simplified schematic diagrams of receiver devices illustrating exemplary embodiments for adjusting DC impedance at the receiver device. In both Figures 9A and 9B, receive antenna 304 feeds an exemplary impedance matching circuit 320 comprising capacitors C1 and C2. The output from impedance matching circuit 320 feeds a simple rectifier 330 (as one example) including diodes D1 and D2 and capacitor C3 for converting the RF frequency to a DC voltage. Of course, many other impedance matching circuits 320 and rectifiers 330 are contemplated to be within the scope of embodiments of the present invention. A DC-DC converter 350 converts the DC input signal 340 from the rectifier into a DC output signal 370 suitable for use by a receiver device (not shown).

Figure 9A illustrates a simple apparatus for maintaining optimum power point impedance in a wireless power transmission system. A comparator 348 compares the DC input signal 340 to a reference voltage 345 selected such that for a given expected power, the impedance seen by the transmitter will result in the maximum amount of power coupled to the DC output signal 370 . The output 361 of the comparator 348 feeds a signal to the DC-DC converter 350 indicating whether the DC-DC converter 350 should increase or decrease its input DC impedance. In embodiments using a switching DC-DC converter 350, this output 361 of the comparator may be converted into a pulse width modulated (PWM) signal that adjusts the input DC impedance (as explained below). This input voltage feedback circuit regulates the input DC impedance by increasing the PWM pulse width as the voltage increases, thus reducing the impedance and voltage.

Figure 9B illustrates a slightly more complex apparatus for maintaining optimal power point impedance in a wireless power transmission system. In FIG. 9B , a current sensor 344 may be included, and a multiplexer 346 may be used to switch whether the voltage or current at the DC input signal 340 is sampled by the processor 360 at any given time. In this system, the voltage (Vr) and current (Ir) of the DC input signal 340 are measured, and the PWM signal 362 to the DC-DC converter 350 can be varied within pre-allowed ranges. The processor 360 can determine which pulse width produces the maximum power (ie, current times voltage) for the PWM signal 362 , which indicates the best DC input impedance. This determined pulse width can be used to deliver an optimal amount of power to the operation of the DC output signal 370 . This sampling and adjustment process can be repeated as often as necessary to track changing coupling ratios, transmit power, or transmit impedance.

As stated earlier, in order to obtain maximum external power from a source with finite output resistance or impedance, the resistance or impedance of the receiver should be the same as that of the source. In many cases, there is a need to operate wireless power systems so as to maximize received power in order to make optimal use of limited RF power.

This maximized power transfer is not always the same as maximum efficiency. In many cases, it may be advantageous to operate the load at a higher (rather than equal) impedance or resistance in order to increase the efficiency of the system. In either case, however, maintaining a certain impedance at the receiver can be useful for adjusting the amount of power transferred between the transmitter and receiver.

In a simple wireless power system, there may be no control over the input impedance; the output load (typically a battery or wireless device) may be the only driver of the system's impedance. This results in suboptimal transmitter/receiver impedance matching, and consequent loss of power transfer, efficiency, or a combination thereof.

DC impedance is defined by (voltage/current). Thus, at any given current and desired impedance, there exists: desired voltage = (current * desired impedance). With a PWM converter, this desired voltage (and thus the desired impedance) can be achieved by providing a feedback term that compares the input voltage to the (current*desired impedance) term, and adjusts the pulse width up or down to maintain that item.

10A-10D show simplified schematic diagrams of receiver devices illustrating exemplary embodiments for adjusting DC impedance at the receiver device using a pulse width modulation converter. In FIGS. 10A to 10D , common elements include receive antenna 304 feeding impedance matching circuit 320 . The output from impedance matching circuit 320 feeds a simple rectifier, shown simply as diode D3. Of course, many other impedance matching circuits 320 and rectifiers are contemplated to be within the scope of embodiments of the present invention. A DC-DC converter 350 converts the DC input signal 340 from the rectifier into a DC output signal 370 suitable for use by a receiver device (not shown). Processor 360 samples parameters of DC input signal 340, DC output signal 270, or a combination thereof, and generates PWM signal 362 for DC-DC converter 350.

The DC-DC converter 350 is a switched mode converter in which the PWM signal 362 controls the switch S1 to periodically charge the filter circuit including the diode D4, the inductor L1 and the capacitor C4. Those skilled in the art will recognize that DC-DC converter 350 is a buck converter that converts the voltage on DC input signal 340 to a lower voltage on DC output signal 370 . Although not shown, those skilled in the art will also recognize that the switched mode DC-DC converter 350 can also be implemented as a boost converter to produce a DC output signal at a voltage higher than the voltage on the DC input signal 340 370.

In most cases, the requirement to regulate the output voltage of the wireless power receiver will be most important. For battery charging, for example, it is often critical not to exceed the maximum output current or maximum output voltage. This means that the output voltage control term will often dictate the control law for the pulse width of the PWM signal 362 .

In many cases, however, the battery will accept power at a rating less than its maximum rating. As an example, during charging of a Li-Ion battery with a rating less than the Li-Ion battery's rated capacity, the voltage will be below the maximum battery voltage and the current may be limited by the maximum power available from the wireless power system. During these conditions, the secondary impedance-control term will become dominant in adjusting the pulse width of the PWM signal in order to control the DC impedance.

Exemplary embodiments of the present invention provide DC impedance control by using a feedback term in the switched mode DC-DC converter 350 to effectively simulate the steady state DC resistance in the receiver. In other words, the DC impedance is controlled by adjusting the frequency or duty cycle of the PWM signal 362 to the switched mode DC-DC converter 350 to simulate a given DC impedance.

Feedback to the system is generated by the processor 360 sampling one or more characteristics of the DC input signal 340, the DC output signal 370, or a combination thereof. Processor 360 then uses this sampled information (possibly along with other information such as the expected power transfer and efficiency of DC-DC converter 350) to adjust PWM signal 362, which adjusts the DC input signal and the DC output signal to close the feedback loop road.

Individual differences in what is sampled and in the way the parameters of the PWM signal are generated are discussed with reference to four different exemplary embodiments as illustrated in FIGS. 10A through 10D .

In FIG. 10A , processor 360 samples the voltage of DC input signal 340 , the current of DC input signal 340 , the voltage of DC output signal 370 , and the current of DC output signal 370 .

In some embodiments, a voltage sensor 342 may be used between the DC input signal 340 and the processor 360 . Similarly, a voltage sensor 372 may be used between the DC output signal 370 and the processor 360 . In other embodiments, voltage sensors 342 and 372 may not be required, and processor 460 may directly sample the voltage on DC input signal 340 and DC output signal 370.

In some embodiments, a current sensor 344 may be used between the DC input signal 340 and the processor 360 . Similarly, a current sensor 374 may be used between the DC output signal 370 and the processor 360 . In other embodiments, current sensors 344 and 374 may not be required, and processor 360 may directly sample the current on DC input signal 340 and DC output signal 370 .

From the current and voltage measurements on both the DC input signal 340 and the DC output signal 370, the processor 360 can determine all parameters required by the power conversion system. The input power on the DC input signal 340 may be determined as the input voltage times the input current. The output power on the DC output signal 370 may be determined as the output voltage times the output current. The efficiency of the DC-DC converter 350 can be determined as the difference between the output power and the input power. The DC impedance of the DC input signal 340 may be determined as the input voltage divided by the input current.

Processor 360 may periodically sample all inputs (eg, approximately once every second, or other suitable period) to determine power output at that time. In response, processor 360 may vary the duty cycle of PWM signal 362 , which will vary the DC impedance of DC input signal 340 . For example, a narrow pulse width on the PWM signal 362 allows the input voltage to remain relatively high and the input current to remain relatively low, which results in a higher DC impedance of the DC input signal 340 . Conversely, a wider pulse width on PWM signal 362 allows more current to be drawn from DC input signal 340 , resulting in a lower input voltage and lower DC impedance for DC input signal 340 .

Periodic sampling and adjustment creates a feedback loop that can find the optimum DC impedance for the DC input signal 340 (and thus the optimum power for the DC output signal 370). The details of finding these values are discussed below with reference to FIG. 11 .

In FIG. 10B , processor 360 samples the voltage of DC input signal 340 , the voltage of DC output signal 370 , and the current of DC output signal 370 . As explained above with reference to FIG. 10A , voltage sensor 342 , voltage sensor 372 , and current sensor 374 may be included between their respective signals and processor 360 , depending on the embodiment.

As in FIG. 10A , in FIG. 10B the output power on the DC output signal 370 can be determined as the output voltage times the output current. In many cases, the efficiency of the DC-DC converter 350 will be known and relatively constant over the desired operating range. Accordingly, processor 360 may estimate the input power of DC input signal 340 based on the output power and an estimate of the efficiency at the current operating point of DC-DC converter 350 . From the estimated input power and the measured input voltage, the DC impedance of the DC input signal 340 can be determined. Again, periodic sampling and scaling creates a feedback loop that can find the optimum DC impedance for the DC input signal 340 (and thus the optimum power for the DC output signal 370).

In FIG. 10C , the processor 360 samples the voltage of the DC input signal 340 and the current of the DC input signal 340 . As explained above with reference to Figure 10A, a voltage sensor 342 and a current sensor 344 may be included between the DC input signal 340 and the processor 360, depending on the embodiment.

In FIG. 1OC, the input power on the DC input signal 340 can be determined as the input voltage times the input current, and the DC impedance of the DC input signal 340 can be determined as the input voltage divided by the input current. As in FIG. 10B , in FIG. 10C the efficiency of the DC-DC converter 350 will be known and relatively constant over the desired operating range. Accordingly, processor 360 may estimate the output power on DC output signal 370 based on the input power and an estimate of the efficiency at the current operating point of DC-DC converter 350 . Again, periodic sampling and scaling creates a feedback loop that can find the optimum DC impedance for the DC input signal 340 (and thus the optimum power for the DC output signal 370).

In FIG. 10D , the processor 360 samples only the voltage of the DC input signal 340 . As explained above with reference to Figure 1OA, a voltage sensor 342 may be included between the DC input signal 340 and the processor 360, depending on the embodiment.

In FIG. 10D , a predetermined estimate may be made for the amount of power expected to be received via the receive antenna and rectifier and delivered on the DC input signal. Using this predetermined estimate, the DC impedance of the DC input signal 340 may be determined relative to the input voltage. As in FIG. 10B , in FIG. 10C the efficiency of the DC-DC converter 350 will be known and relatively constant over the desired operating range. Accordingly, processor 360 may estimate the output power on DC output signal 370 based on the predetermined input power estimate and an estimate of the efficiency at the current operating point of DC-DC converter 350 . Again, periodic sampling and scaling creates a feedback loop that can find the optimum DC impedance for the DC input signal 340 (and thus the optimum power for the DC output signal 370).

The predetermined power estimate may be a fixed value programmed into the receiver device, or may be communicated to the receiver device from a transmitter device which may have a function for determining how much of the transmitted power will be coupled to the receiver device. device for that particular receiver device.

11 illustrates various input and output parameters that may be used in adjusting the DC impedance at the receiver device. This graph represents a system with a specific source impedance, but where the load resistor is allowed to vary over a wide range. This load resistor may be represented as a variable resistor of the DC-DC converter 450 of FIG. 6 . Alternatively, the load resistor may be represented by the DC impedance of the DC input signal 340 to the DC-DC converter 350 shown in FIGS. 9A-10D .

In FIG. 11 , a source impedance of 50 ohms is driven by a signal with a 1:1 source-load coupling. Line 620 shows the current through the load resistor. Note that as the load impedance increases, the current decreases due to Ohm's law. Line 610 shows the voltage across the load resistor. Note that as the load impedance increases, the voltage also increases equally divided by the resistor divider.

These two data sets for the current and voltage of the load resistor give the power across the load resistor (as shown by line 640 ). Note that the power peaks at a certain load impedance. In this case (1:1 load coupling), this maximum power point occurs when the load impedance is equal to or close to the source impedance. The peak power point may also be shifted if the coupling is different.

Line 650 represents the PWM setting (outside 100), which is inversely related to output impedance. This is the function exhibited by most buck converters. As can be seen, there is an ideal PWM setting that maximizes the power received by the load resistor. The wireless power impedance control scheme used with reference to the exemplary embodiments discussed herein attempts to find and maintain this ideal PWM setting.

Of course, as stated earlier, optimal power transfer is not always necessary. Using the embodiments of the invention discussed above in FIGS. 6 and 9A-10D , the DC impedance of the DC input signal 340 (and thus the AC impedance of the receive antenna) can be effectively detuned from optimal power transfer. , to limit the amount of power delivered on the DC output signal 370 . This power limitation may be useful in cases where the receiver device cannot accept the maximum power deliverable from the DC-DC converter 350 . Some non-limiting examples of this need for reduced power may occur when the battery in the receiver device is near full charge or when the DC-DC converter 350 can deliver more power than the battery's rated capacity.

Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or magnetic particles, optical fields or optical particles, or any combination thereof and chips.

Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the exemplary embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the exemplary embodiments of the present invention.

A general-purpose processor, digital signal processor (DSP), application-specific integrated circuit (ASIC), field-programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or its design to perform Any combination of the functions described herein implements or performs the various illustrative logical blocks, modules, and circuits described in connection with the exemplary embodiments disclosed herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, eg, a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

The steps of methods or algorithms described in conjunction with the exemplary embodiments disclosed herein may be directly embodied in hardware, in software modules executed by a processor, or in a combination of the two. Software modules can reside in random access memory (RAM), flash memory, read only memory (ROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), registers, hard disk, removable diskette, CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral with the processor. The processor and storage medium can reside in the ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and storage medium may reside as discrete components in the user terminal.

In one or more exemplary embodiments, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. Storage media may be any available media that can be accessed by a computer. By way of example and not limitation, such computer readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage, or may be used to carry instructions or data structures or any other medium that stores desired program code and can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then coaxial cable, fiber optic Cable, twisted pair, DSL or wireless technologies such as infrared, radio and microwave are included in the definition of media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers Reproduce data. Combinations of the above should also be included within the scope of computer-readable media.

The previous description of the disclosed exemplary embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these exemplary embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (31)

1. A wireless power receiver comprising: a rectifier coupled to an antenna configured to receive a wireless power signal, the rectifier configured to convert the wireless power signal into a DC input signal; a DC-DC converter configured to generate a DC output signal based on the DC input signal and a pulse width modulated signal; and A pulse modulator configured to adjust an AC impedance of the wireless power receiver by modifying a duty cycle of the pulse width modulated signal to the DC-DC converter based on the DC input signal. 2. The wireless power receiver of claim 1, wherein the DC-DC converter is further configured to adjust a power output of the DC output signal based on the DC input signal. 3. The wireless power receiver of claim 1, wherein the pulse modulator is configured to modify the pulse width based on at least one of a current of the DC input signal and a voltage of the DC input signal Modulated signal. 4. The wireless power receiver of claim 1 , wherein the pulse modulator is configured to modify the pulse width based on at least one of a voltage of the DC output signal and a current of the DC output signal Modulated signal. 5. The wireless power receiver of claim 1 , wherein the pulse modulator further comprises a comparator configured to generate the pulse width based on a comparison of the DC input signal with a voltage reference signal Modulated signal. 6. The wireless power receiver of claim 1, wherein the pulse modulator further comprises a processor configured to: receiving the DC input signal; sampling a value of the DC input signal; and The AC impedance of the wireless power receiver is adjusted, wherein adjusting includes adjusting the duty cycle of the pulse width modulated signal based on the value sampled from the DC input signal to substantially maximize power output. 7. The wireless power receiver of claim 6, wherein the sampled value is one or more of a current of the DC input signal and a voltage of the DC input signal. 8. The wireless power receiver of claim 7, wherein the processor is further configured to: sampling at least one of a voltage of the DC output signal and a current of the DC output signal; and The duty cycle of the pulse width modulated signal is further adjusted based on the sampled voltage of the DC output signal, the sampled current of the DC output signal, or a combination thereof. 9. The wireless power receiver of claim 6, further comprising: an input current sensor configured to generate an input current sense signal based on a current of the direct current input signal, and wherein the processor is further configured to: receiving the input current sense signal; and sampling one or more values from the input current sense signal, wherein adjusting the AC impedance of the wireless power receiver further comprises adjusting the duty cycle of the pulse width modulated signal based on the one or more values sampled from the input current sense signal to substantially The power output on the DC output signal is maximized. 10. The wireless power receiver of claim 6, further comprising: an output current sensor configured to generate an output current sense signal based on a current of the direct current output signal, and wherein the processor is further configured to: receiving the output current sensing signal; sampling a value of the output current sense signal; and Sampling the voltage of the DC output signal; wherein adjusting the AC impedance of the wireless power receiver further comprises: adjusting the pulse width modulation signal based on the value sampled from the output current sense signal and a sampled voltage of the DC output signal to The power output on the DC output signal is substantially maximized. 11. The wireless power receiver of claim 1, wherein the pulse modulator further comprises a processor configured to: receiving the DC input signal; and sampling the value of the DC input signal; wherein modifying the duty cycle of the pulse width modulated signal comprises modifying the duty cycle of the pulse width modulated signal based on the values sampled from the dc input signal to convert The power output is reduced to a power level that is less than an optimum power level for a receiver device configured to receive the direct current output signal and acceptable to the receiver device. 12. The wireless power receiver of claim 1, wherein the DC-DC converter comprises a buck converter or a boost converter, the buck converter or the boost converter configured to receive the dc input signal, the dc output signal and the pulse width modulated signal, the dc-dc converter configured to: adjusting input impedance on said DC input signal; and The voltage of the DC output signal is adjusted relative to the DC input signal based on the duty cycle of the PWM signal. 13. A method for impedance tuning in wireless power transmission, comprising: Receive wireless power signals; rectifying the wireless power signal into a DC input signal; converting the DC input signal to a DC output signal; and An AC impedance of the wireless power receiver is modified by adjusting a power output of the DC output signal based on the DC input signal. 14. The method of claim 13 , wherein modifying the AC impedance comprises adjusting a voltage of the DC output signal based on one or more of a voltage of the DC input signal and a current of the DC input signal. the power output. 15. The method of claim 13 , wherein modifying the AC impedance comprises adjusting a voltage of the DC output signal based on one or more of a voltage of the DC output signal and a current of the DC output signal. the power output. 16. The method of claim 13, wherein modifying the AC impedance comprises based on the voltage of the DC input signal, the current of the DC input signal, the voltage of the DC output signal, and the current of the DC output signal One or more of them to adjust the power output of the DC output signal from a DC-DC converter. 17. The method of claim 13, wherein said modifying said AC impedance is based on a comparison of said DC input signal with a voltage reference signal. 18. The method of claim 13, wherein converting the DC input signal to the DC output signal comprises buck converting or boost converting, the converting further comprising: sampling the voltage of the DC input signal; determining a maximum power output based on the sampled voltage of the DC input signal; and A pulse width modulated signal is adjusted to substantially maximize the power output of the DC output signal. 19. The method of claim 13, wherein converting the DC input signal to the DC output signal comprises buck converting or boost converting, the converting further comprising: sampling the voltage of the DC input signal; determining a maximum power output based on the sampled voltage of the DC input signal; and adjusting a pulse width modulated signal to reduce a power output on the direct current output signal to a power level that is less than an optimum power level for a receiver device configured to receive the direct current output signal and acceptable for the receiver device. 20. The method of claim 13, wherein converting the DC input signal to the DC output signal comprises buck conversion or boost conversion, the converting further comprising: sampling one or more values from at least one of the voltage of the DC input signal, the current of the DC input signal, the voltage of the DC output signal, and the current of the DC output signal; and A maximum power output is determined based on one or more of the sampled values. 21. A wireless power receiver comprising: Devices for receiving wireless power signals; means for rectifying said wireless power signal into a DC input signal; means for converting said DC input signal into a DC output signal; and means for modifying an ac impedance of the wireless power receiver comprising adjusting the dc output from the means for converting the dc input signal into a dc output signal based on the dc input signal Signal power output of the device. 22. The wireless power receiver of claim 21 , wherein said means for modifying said AC impedance of said wireless power receiver further comprises means for modifying said DC input signal based on a voltage of said DC input signal and said DC input means for adjusting said power output of said dc output signal from said means for converting said dc input signal into a dc output signal by one or more of the currents of the signal. 23. The wireless power receiver of claim 21 , wherein said means for modifying said AC impedance of said wireless power receiver further comprises means for modifying a voltage based on said DC input signal, said DC input signal current, voltage of the DC output signal, and current of the DC output signal to adjust the current from the means for converting the DC input signal into a DC output signal DC output signal of the power output device. 24. The wireless power receiver of claim 21 , further comprising means for comparing the DC input signal with a voltage reference signal to determine a value, the means for modifying the The means for AC impedance is configured to modify the AC impedance based on the value. 25. The wireless power receiver of claim 21 , wherein said means for converting said DC input signal to a DC output signal comprises means for buck conversion or means for boost conversion At least one, and wherein said means for converting said DC input signal into said DC output signal further comprises: means for sampling one or more values from said DC input signal; means for determining a maximum power output based on a sampled voltage of said DC input signal; and means for adjusting a pulse width modulated signal received by said means for converting said direct current input signal into said direct current output signal to substantially maximize said power output of said direct current output signal. 26. The wireless power receiver of claim 25, wherein the sampled one or more values are one or more of a current of a DC input signal and a voltage of a DC input signal. 27. The wireless power receiver of claim 25, wherein said means for modifying an AC impedance of said wireless power receiver, said means for sampling one or more values from said DC input signal , said means for determining said maximum power output based on a sampled voltage of said DC input signal and said means for adjusting said means for converting said DC input signal into said DC output signal Means for receiving said pulse width modulated signal to substantially maximize said power output of said DC output signal are included within processor means. 28. The wireless power receiver of claim 21 , wherein said means for converting said DC input signal to a DC output signal comprises means for buck conversion or means for boost conversion At least one, and wherein said means for converting said DC input signal into said DC output signal further comprises: means for sampling the voltage of said DC input signal; for adjusting a pulse width modulated signal received by said means for converting said DC input signal into said DC output signal to reduce said power output on said DC output signal to a power level device, the power level is less than an optimum power level for and acceptable to a receiver device configured to receive the direct current output signal. 29. The wireless power receiver of claim 21 , further comprising: means for sampling one or more values from at least one of the voltage of the DC input signal, the current of the DC input signal, the voltage of the DC output signal, and the current of the DC output signal; and Means for determining maximum power output based on sampled values. 30. A method of impedance tuning in wireless power transmission, comprising: coupling near-field radiation in the coupling mode region to generate a wireless power signal for a wireless power receiver; rectifying the wireless power signal into a DC input signal; converting the DC input signal to a DC output signal; and An AC impedance of the wireless power receiver is modified by adjusting a power output of the DC output signal from a DC-DC converter based on the DC input signal. 31. The method of claim 30, wherein modifying the AC impedance of the wireless power receiver comprises: based on the voltage of the DC input signal, the current of the DC input signal, the voltage of the DC output signal and one or more of the current of the DC output signal to adjust the power output of the DC output signal from a DC-DC converter.